Eddy current inspection probe based on magnetoresistive sensors

ABSTRACT

A device and method of eddy current based nondestructive testing of tubular structures made of electrically conductive materials is disclosed. The device includes a plurality of excitation electromagnets sensors having an easy axis for magnetic field sensing; wherein the magnetoresistive sensors are arranged in a circular array on a single plane with the easy axis aligned radially with respect to the circular pattern and wherein the electromagnets are arranged in a circular pattern on both sides of the plane with their axes of symmetry being arranged parallel to the plane and orthogonal to radii of the circular pattern on which the electromagnets are placed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This utility application claims the benefit under 35 U.S.C. §119(e) of provisional application Ser. No. 61/813,899 filed on Apr. 19, 2013, and entitled Eddy Current Inspection Probe for Imaging Magnetic Flux Leakage of Flaws and Points of

Interest in Tubular Structures Based on Magnetoresistive Sensors. The entire disclosure of this provisional application is included herein by reference.

FIELD OF THE INVENTION

The invention is directed to sensor probes for eddy current non-destructive testing and, in particular, to such probes employing magnetoresistive sensors.

BACKGROUND OF THE INVENTION

Nondestructive evaluation (NDE) technologies have been recently challenged to find material defects such as fatigue flaws, cracks and damage precursors such as stress or corrosion induced local conductivity variation in structures with higher probability of detection (POD) and a level of improvement is necessary as these issues are critical to operational safety. Certain inspection opportunities and their specific geometries often necessitate off-the-surface or non-contact methodologies thereby eliminating methods such as ultrasound testing where either physical contact or a transmit medium necessary for inspection. The non-destructive testing industry currently does most non-contact inspection of conductive materials via conventional wound inductive coil based eddy current inspection techniques. These inspections have limited spatial resolution due to sense coil size as well as frequency dependent sensitivity and thus have limited efficacy.

Eddy current testing (ECT) probes to locate and characterize flaws or material defects in a conductive material are known. An ECT probe does this by sensing the out-of-plane magnetic flux leakage (MFL) created by the deviation of eddy currents by the flaws or defects in the area under test (AUT).

Technological advancements in the manufacturing of these elements have led to commercially accessible sensing elements. Low cost anisotropic magnetoresistance AMR and giant magnetoresistive GMR magnetometers (referred to collectively as “XMR” sensors herein) are now available which are sensitive, have small package size, consume little power, and operate at room temperature.

ECT utilizing XMR sensing can have a higher level of utility, as these sensing elements are non-inductive and orders of magnitude smaller than traditional eddy current coils. A magnetoresistive (MR) sensor is a solid-state device that utilizes electron conduction physics to convert a magnetic field into an electrical signal. Anisotropic magnetoresistance (AMR), for example, is a solid-state sensing element that has a permalloy (Ni80-Fe20) electrodeposited line on silicon for sensing low-level magnetic fields. This occurs by an alignment of the material's magnetic domains in response to the external magnetic field of interest. This magnetic domain alignment changes the resistivity of the sensor via induced changes in the scattering matrix (spin-coupled interaction between the conduction electrons and the magnetic moments in the material)). In contrast a magnetic sensor exhibiting the giant magnetoresistive (GMR) mechanism will convert a sensed magnetic field to an electrical signal is exploiting the spin-coupled charge interaction of a multi-layer structure. This structure is a three-ply stack of a ferromagnetic material (FM), a non-magnetic conductive layer (NM) and a bottom layer of ferromagnetic material (FM) all on a silicon substrate.

One of the challenges with XMR sensors is that they have no means of discriminating magnetic fields sensed along the easy axis. Because the level of the field of interest will be orders of magnitude lower than the background drive magnetic field, it has been historic precedent to either: (a) shield the sensor from the drive field or (b) orient the sensor such that the sensitive axis is orthogonal to the drive field as to not saturate the sensing element. For example, one can find the use of concentric/co-located sensors and drives in US patents: U.S. Pat. No. 6,888,346, 2011/0068784 A1, 2005/0007108 A1, U.S. Pat. No. 6,888,346 as well as 2005/0007108 A1. All of the documents cited herein are incorporated by reference in their entireties. This has led to the vast majority of embodied XMR based ECT probes towards using the sensor in a horizontal sensing configuration with respect to the AUT while positioned in the center of an excitation coil. As illustrated in FIG. 1, this allows the sensor 10 to be co-located with the drive coil 20 and positioned in the bore of the coil orienting the excitation field B_(EX) orthogonally to the sensor easy axis. Therefore the sensor would be immune to the excitation field.

This configuration of XMR 10 to the surface of the AUT 30 does not lead to the same signal morphology in response to a material defect 31 as a wound pancake inductive coil ECT probe. Signal morphology is a critical ECT product requirement as there is often continuity required with historic inspection data. A pancake coil based ECT probe does a spatial integration of the time rate of change of all three axes of magnetic flux leakage at any point in space created by perturbation of the eddy current distribution by a discontinuity in the material. Because the largest vector component B_(ZMFL) of the of the out-of-plane MFL will be the component orthogonal to the AUT, it is the most dominant component in the coils' spatial integration and thus influences most the eddy current signal response. This is best approximated by vertical sensing methodologies (in Cartesian coordinates or radial in cylindrical coordinates) that align the easy axis of the XMR sensor with this field component of the MFL as shown in FIG. 2.

Because of the aforementioned reasons, to date, there has not been a practicable ECT probe that orients an XMR sensor with the easy axis aligned orthogonal to the surface of the AUT.

SUMMARY OF THE INVENTION

In an embodiment of the invention there is device for eddy current based nondestructive testing of tubular structures made of electrically conductive materials wherein the device includes: a plurality of excitation electromagnets having an axis of symmetry and magnetoresistive sensors having an easy axis for magnetic field sensing,

The magnetoresistive sensors are arranged in a circular array on a single plane with said the axis aligned radially with respect to the circular pattern and the electromagnets are arranged in a circular pattern on both sides of the plane with their axes of symmetry being arranged parallel to the plane and orthogonal to radii of the circular pattern on which the electromagnets are placed. In a further embodiment, the magnetoresistive sensors are either anisotropic magnetoresistive or giant magnetoresistive sensing elements. In a further embodiment, the excitation electromagnets are wound on coils having a substantially rectangular cross section with four faces and wherein the electromagnets are arranged with one of their faces aligned along at the perimeter of the circular pattern. In a further embodiment, the sensors are mounted between a pair of parallel circular printed circuit boards, the electromagnets are mounted on opposite faces of the printed circuit boards from the sensors and electrical connections to the sensors and the electromagnets are made through the printed circuit boards. In a further embodiment, the excitation electromagnets are arranged to be energized individually. In a further embodiment, a subset of the excitation electromagnets are excited together to create a continuous azimuthally orientated eddy current distribution an area under test. In a further embodiment, a subset of the excitation electromagnets are excited together and a second subset are not excited so as to create an axially oriented eddy current distribution to an area under test. In a further embodiment, the electromagnets are arranged in a substantially staggered pattern such that no two electromagnets on either side of said circular plane are directly opposite each other.

In an embodiment of the invention there is a device for eddy current based nondestructive testing of an article made of electrically conductive materials, which includes: a plurality of excitation electromagnets, each having an axis of symmetry and magnetoresistive sensors having an easy axis for magnetic field sensing. The excitation electromagnets are arranged to induce an eddy current in the article and the magnetoresistive sensors are arranged such that the easy axis is substantially orthogonal to the axis of symmetry. In a further embodiment, the excitation electromagnets are wound on coils having a substantially rectangular cross section with four faces and wherein said electromagnets are arranged with one of their faces aligned to be substantially parallel to a surface of the article, while the sensor are arranged with their easy axes substantially orthogonal do the surface. In a further embodiment, the magnetoresistive sensors are either anisotropic magnetoresistive or giant magnetoresistive sensing elements.

In an embodiment of the invention there is a method of non-destructively testing an article made of electrically conductive material. The method includes the steps of: inducing an eddy current in the article, the eddy current having a direction; sensing the eddy current with either anisotropic magnetoresistive or giant magnetoresistive sensing elements, the sensing elements having an easy axis for magnetic field sensing; and aligning the sensing elements with the easy axis substantially orthogonal to the eddy current direction so as to sense any magnetic flux caused by flaws in the article. In a further embodiment, the article is a tube and the direction is either an axial direction or a circumferential direction. In a further embodiment, the sensing elements in a circular pattern in one plane at the perimeter of a cylindrical probe with said easy axis arranged in a radial direction. Ina further embodiment, the excitation coils are arranged around the perimeter on either side of the plane of sensing elements. 

What is claimed is:
 1. A device for eddy current based nondestructive testing of tubular structures made of electrically conductive materials comprising: a plurality of excitation electromagnets having an axis of symmetry and magnetoresistive sensors having an easy axis for magnetic field sensing; wherein said magnetoresistive sensors are arranged in a circular array on a single plane with said easy axis aligned radially with respect to said circular pattern and wherein said electromagnets are arranged in a circular pattern on both sides of said plane with their axes of symmetry being arranged parallel to said plane and orthogonal to radii of said circular pattern on which said electromagnets are placed.
 2. The device of claim 1, wherein said magnetoresistive sensors are either anisotropic magnetoresistive or giant magnetoresistive sensing elements.
 3. The device of claim 1, wherein said excitation electromagnets are wound on coils having a substantially rectangular cross section with four faces and wherein said electromagnets are arranged with one of their faces aligned along at the perimeter of said circular pattern.
 4. The device of claim 1, wherein said sensors are mounted between a pair of parallel circular printed circuit boards, said electromagnets are mounted on opposite faces of said printed circuit boards from said sensors and wherein electrical connections to said sensors and said electromagnets are made through said printed circuit boards.
 5. The device of claim 1, wherein said excitation electromagnets are arranged to be energized individually.
 6. The device of claim 1, wherein a subset of said excitation electromagnets are excited together to create a continuous azimuthally orientated eddy current distribution an area under test.
 7. The device of claim 1, wherein a subset of said excitation electromagnets are excited together to create an axially oriented eddy current distribution to an area under test.
 8. The device of claim 1, wherein said electromagnets are arranged in a substantially staggered pattern such that no two electromagnets on either side of said circular plane are directly opposite each other.
 9. A device for eddy current based nondestructive testing of an article made of electrically conductive materials comprising: a plurality of excitation electromagnets each having an axis of symmetry and magnetoresistive sensors having an easy axis for magnetic field sensing; wherein said excitation electromagnets are arranged to induce an eddy current in the article and wherein said magnetoresistive sensors are arranged such that said easy axis is substantially orthogonal to said axis of symmetry.
 10. The device of claim 9, wherein said excitation electromagnets are wound on coils having a substantially rectangular cross section with four faces and wherein said electromagnets are arranged with one of their faces aligned to be substantially parallel to a surface of the article.
 11. The device of claim 9, wherein said magnetoresistive sensors are either anisotropic magnetoresistive or giant magnetoresistive sensing elements.
 12. A method of non-destructively testing an article made of electrically conductive material comprising: inducing an eddy current in said article, said eddy current having a direction; and sensing said eddy current with either anisotropic magnetoresistive or giant magnetoresistive sensing elements, said sensing elements having an easy axis for magnetic field sensing; and aligning said sensing elements with said easy axis substantially orthogonal to said eddy current direction so as to sense any magnetic flux caused by flaws in the article.
 13. The method of claim 12 wherein said article is a tube and said direction is either an axial direction or a circumferential direction.
 14. The method of claim 13, further comprising arranging said sensing elements in a circular pattern in one plane at the perimeter of a cylindrical probe with said easy axis arranged in a radial direction.
 15. The method of claim 14, further comprising arranging excitation coils around said perimeter on either side of said plane of sensing elements. 